A day after federal health officials tied an outbreak of cyclospora to shredded iceberg lettuce supplied by the large produce company Taylor Farms to Taco Bell, investigators are turning their attention to the farms in Mexico where it was grown and the facility that shredded it, according to administration officials.

Among their most urgent questions is whether contaminated lettuce from the growers and processor has been shipped to other food retailers in the United States. Investigators from the Food and Drug Administration were planning to travel to Mexico to visit the farms and shredding facility and scour their records, an official said.

They are also investigating how the lettuce was contaminated by cyclospora. The parasite, which lays eggs that embed in the intestines when they are consumed, can cause explosive diarrhea, cramping, bleeding and other symptoms that can last weeks.

More than 1,644 people reported eating at Taco Bell before becoming sick with the parasitic infection, and 94 have been hospitalized, according to the Centers for Disease Control and Prevention. But dozens of others who had not eaten at Taco Bell have fallen ill from the parasite too.

On Friday afternoon, Taylor Farms said on Facebook that none of their salads or kits were associated with the outbreak. They said they had removed all iceberg lettuce from the Central Mexico region from their products indefinitely.

As the investigation intensifies, Taylor Farms, among the nation’s largest producers of salad greens and vegetables, has hired a former Trump White House official.